Housing for the nacelle of a wind turbine

ABSTRACT

Disclosed is a housing for the nacelle of a wind turbine, comprising a support structure ( 15 ) composed of tubes and braces, preferably made of metal, especially steel, or plastic, especially fiberglass reinforced plastic (FRP), and covering segments ( 2, 3, 4 ) which are fastened to said support structure ( 15 ). The housing is characterized in that the covering elements ( 2, 3, 4 ) have a coating surface made of plastic, especially FRP, and are self-supporting. Such a housing for the nacelle of a wind turbine can be manufactured with little effort from inexpensively produced parts and is easy to assemble. In particular, said housing combines the advantages of previously known structures that are self-supporting as a whole and the paneling of a support structure comprising parts that are not self-supporting without having the disadvantages thereof.

FIELD OF THE INVENTION

The invention relates to a housing for the nacelle of a wind turbine, having the characteristics of the preamble to claim 1.

PRIOR ART

Wind turbines have a tower on which a so-called nacelle, also called a machine house, is rotatably disposed. The rotor is disposed on the face end of this nacelle and is connected to a rotor shaft that protrudes into the nacelle. In the nacelle itself, optionally, there is a gear, as well as the generator used to generate current and possibly further components.

On the one hand for protection against the effects of weather but on the other also to deflect wind loads, the nacelle has to be provided with, or closed with, a covering or a housing.

For that purpose, various approaches and procedures are known from the prior art.

For instance, self-supporting constructions of plastic, typically FRP, are known, which comprise a few segments, such as two lower parts and a cap. However, the size of the components, and the associated great imprecision with regard to geometric tolerances, which are systematic in FRP construction, are both problematic. Also, relatively large-volume support structures must be constructed in the laminate for the function of the self-supporting structures, and given the tight space for the nacelles, this often leads to structural restrictions. Moreover, shipping such large-sized elements is expensive, since as a rule the coverings comprise only a few large elements (often only one component). The special shipments thus necessary are logistically complicated and entail high costs.

Self-supporting constructions of steel or aluminum with an internal structure or self-supporting shells are also known. The structures are typically quite simple and structurally favorably constructed. Metal (including aluminum) plates, however, as two-dimensional material, are not especially well suited from an economic standpoint, because of the relatively high density and costs (especially for aluminum).

Finally, supporting steel structures for the construction of nacelle housings or sheaths which are paneled are known. That is, in these steel structures, non-self-supporting two-dimensional elements of FRP or a metal material are secured directly, using a large number of rivets or screws. The result in these known systems is a large number of separation points, which are complicated in terms of production, since they have to be made tight against the effects of weather. Experience shows that the costs for the system rise markedly if systematic sealing between the segments is to be attained, since in that case high degrees of precision and high expense for assembly are required. Moreover, shipping the non-self-supporting two-dimensional elements is expensive; they have to be shipped in complicated, space-hogging holder structures.

One example for a nacelle housing constructed of fiber reinforced plastic elements is disclosed in German Patent Disclosure DE 10 2006 001 931 A1. The example disclosed involves a self-supporting construction of fiber reinforced plastic which is complicated in its construction. Moreover, the individual components of the construction shown here, such as a side wall covering, are quite large, which leads to problems in adhering to tolerances, on the one hand, and in shipping these parts, on the other.

One example for a housing of a nacelle of a supporting steel structure, which is paneled for constructing the sheath, is described in German Patent Disclosure DE 10 2005 042 394 A1. There, the nacelle housing first has a trellis-like steel frame structure, onto which individual outer skin covering elements are then screwed or secured in some other way.

In International Patent Disclosure WO 2007/132408 A2, finally, a further housing for the nacelle of a wind turbine is shown, constructed of individual large-area segments.

SUMMARY OF THE INVENTION

In this respect it is the object of the invention to disclose a housing for the nacelle of a wind turbine which can be produced from parts that are economical to produce and at little production costs and can be erected easily.

This object is attained with a housing for the nacelle of a wind turbine having the characteristics of claim 1. Advantageous refinements of the housing of the invention are recited in dependent claims 2-15.

According to the invention, a housing, which can also be called a sheath or outer wall, for the nacelle, which is also called a machine house, in a wind turbine is first embodied as a non-self-supporting construction. Specifically, according to the invention, a support structure of tubes or braces, preferably of metal and in particular steel, or of plastic and in particular FRP, is provided as a “substructure”, on which covering elements are then affixed. To this extent, the procedure of the invention and the housing of the invention are similar to the construction described above in the prior art of non-self-supporting housings. The essential distinction from the above-described prior art, however, is the covering elements to be provided according to the invention. Unlike in the prior art, where simple, non-self-supporting metal planks or plastic elements are used as covering elements, in the invention covering segments are employed, which have a covering surface of plastic, in particular FRP, and are embodied as self-supporting.

“Self-supporting” in the sense of the invention includes in particular the fact that after the covering segments have been assembled the dimensioning loads from wind and traffic are picked up from these segments and can be transmitted to the support structure via what is kept as a limited number of connection points. In this respect, the structure of the invention differs from the paneled versions with non-self-supporting elements, which in the final analysis must be connected to the support structure along the entire circumference to make it possible for loads to be absorbed and transmitted onward.

Thus in a sense, two concepts known from the prior art but until now always pursued differently, are in a sense combined in a skilled and intrinsically surprising manner, namely the supporting or support structure from the concept of the load-bearing mode of construction, and the aspect of the self-supporting individual elements, which is known from the structural type, embodied overall as self-supporting, for generic housings. The step, which to one skilled in the art is initially remote and unconventional, of embodying the “paneling” of a load-bearing structure, that is, the support structure of tubes or braces, with load-bearing covering segments at increased engineering expense and material consumption, leads in the final analysis to very pronounced and useful advantages:

-   -   Securing the covering segments to the support structure can be         done at substantially fewer securing points than in the known         paneling, so that considerable labor time is saved, and the         effort and expense for maintenance or repair to the housing can         be reduced.     -   The covering segments can be shipped substantially more easily,         since because of their self-supporting property they need not be         braced in a complicated way or placed in mounting devices. It is         advantageous in this respect if the covering segments are made         in a size that allows them to be packaged packaging in         conventional shipping containers, typically conventional 40-foot         containers (see claim 15).     -   The self-supporting covering segments need not be secured all         the way around on the support structure, so that the demands for         dimensional stability can be lowered, which particularly in         plastic construction (FRP construction) that involves tolerances         leads to considerable reduction in effort and thus in cost.

The covering segments can fundamentally be embodied as self-supporting in an arbitrary way. However, preferably they are formed from a frame, which may comprise metal tubes or braces, or of plastic, in particular FRP, which holds the component and lends it the actual stability, as well as of a shell element, mounted on the frame face and solidly connected to the frame, which is of plastic that is preferably fiber reinforced, in particular FRP. The thus-fabricated covering segments are then connected in turn to the support structure via the frame in the usual way, for instance in the case of metal frames and a support structure of metal tubes or braces, via usual metal construction means, or in other words are screwed, welded, or the like.

In the case of a frame of plastic, the frame preferably comprises the plastic material of the shell element. Thus no stresses or the like arise in the covering segment, stresses that if different materials were used could be due for instance to different coefficients of thermal expansion.

The stability of the covering segment is brought about by the frame, which in the particular covering segment advantageously extends over a predominant portion of the length and width thereof. Accordingly, the shell element can be designed of plastic, with reduced dimensions in thickness or material thickness; it need not take on any supporting load in the construction itself but instead must merely withstand the snow loads and wind load that occur.

The solid connection according to the invention between the shell element and the frame can advantageously be brought about already in the fabrication of that component; the frame can for instance be laminated into the shell element. The frame can then in turn be affixed on the support structure by simple machine or metal construction means, and a substantially smaller number of connection points is necessary than for instance in the known paneling of simple sheath elements.

The connection between the frame of a covering segment and the support structure can in particular also be designed as detachable, so that individual covering elements can be removed for maintenance purposes and optionally replaced if replacement becomes necessary.

A further advantage of the housing of the invention, if the frames of the covering segments are formed of metal, is that metal components, especially steel components, of the kind formed by the frame can be made with markedly higher precision and dimensional stability than plastic elements. Thus problems of exceeding tolerances and of dimensional stability that otherwise must be encountered do not occur, or such peripheral conditions need not be taken into account.

Connecting the frame to the shell element to form the covering segment can be done for instance by providing that the plastic material of the shell element at least partially or in portions overlaps the tubes or braces or other elements of the frame. What is important here is that those regions of the frame that later serve to attach to the support structure are not covered by the plastic material, but instead are freely accessible. In such a design of the connection between the shell element and the frame, especially if different materials are used for the frame and shell elements, it is advantageous if the connection is made such that relative motion between the tube or brace, or other elements of the frame, and the shell element, particularly relative motion from different thermal expansions, remains possible. Here, a gel coat can for instance also be used, which makes sliding of the frame on the plastic material possible. In that case, corresponding overlapping of the plastic material over the frame should advantageously pertain only to the tubes or braces of the frame extending longitudinally, since it is in that direction that the greatest relative motion, for instance from thermal stresses, is to be expected.

The housing of the invention may advantageously be embodied such that between the covering segments at the abutting point, air gaps can be left, having a width of up to 20 mm and preferably in the range from 10 mm to 15 mm. These air gaps make it possible to adhere to very rough tolerances for constructing the covering segments, and in particular for instance the plastic and especially FRP shell elements, in the production of which greater inaccuracies of measurement can typically occur than in the production of a frame structure for instance of metal located below them that is relevant to the dimensional stability of the covering segment. These air gaps furthermore also serve to a certain extent to supply fresh cold air into the interior of the nacelle and to carry heated air out of the nacelle, so that at least a certain cooling effect can be attained for the components disposed in the nacelle that generate heat.

Advantageously, a first covering segment, disposed in a fall line of the housing above a second covering segment, protrudes with its edge past the edge of the second covering segment. As a result, rainwater running off on the outer wall of the housing is in particular prevented from reaching the interior of the nacelle through the air gaps. As a result of the overlap, the water always runs from the outside of one covering segment onto the outside of the adjoining covering segment; thus water is prevented from penetrating into the interior of the nacelle.

Particularly in regions where the occurrence of sandstorms is a threat, or where considerable proportions of sand or other abrasive, corrosive or otherwise harmful particles, such as salt particles or moisture, are carried with the wind, the air gaps can be partly closed by sealing lips. This prevents the entry of sand or other unwanted particles into the interior of the nacelle, where such sand or such particles can cause damage, for instance from accelerated mechanical wear or from problems in the electronics.

The frames of the covering segments are preferably made as closed, surrounding frames, and especially advantageously from square tubes. Closed encompassing frames are especially stable, and square tubes are especially well suited to attachment to the support structure because they have a flat contact face.

As already noted, the covering segment with its covering surface, such as the shell element, can be formed with essentially smooth surfaces and with a slight thickness that can be less than 10 mm, preferably from 3 to 5 mm, and in an especially preferred exemplary embodiment, 4 mm. Such thin and essentially plane elements of plastic, in particular FRP, can be produced inexpensively and simply; however, as before, they are sufficiently stable to absorb slow loads or wind loads burdening them and transmitting them to the frame or by way of the frame to the support structure.

An especially simple construction of the support structure is obtained if in an advantageous feature of the invention the support structure is formed from a carrier portion, which extends essentially in the longitudinal direction of the nacelle, and from at least two and preferably three portals that span this support structure in the transverse direction of the nacelle. At least the lateral and upper covering segments can then be affixed to these portals. To that end, the portals are likewise preferably made from a square tube or a brace of square profile.

In one possible feature of a housing of the invention, it can contain a total of twelve covering segments, of which two each are mounted on all six sides of the housing. However, still more covering segments may be provided, since advantageously the maximum dimensions of the individual covering segments are selected such that they can be shipped using conventional shipping devices, and in particular such that there is space for them in a conventional 40-foot container, even together with a shipping frame or protection elements that are to be provided.

Besides the advantages described above, for the housing of the invention there is still another advantage in that for the construction of fixtures on the housing, for instance on the outside of the nacelle, connections can be made which extend into the reinforcing structures of the covering segments, for instance into the material comprising the frames, so that simple connections such as screw connections can be made here.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and characteristics of the invention will become apparent from ensuing description of an exemplary embodiment in conjunction with the accompanying drawings.

FIG. 1 is a schematic three-dimensional view obliquely from the front of a housing of the invention for the nacelle of a wind turbine;

FIG. 2 is a three-dimensional view of the housing, shown in FIG. 1, from a different perspective, namely obliquely from below;

FIG. 3 is a schematic view in one exemplary embodiment of a covering segment for constructing a housing of the invention;

FIG. 4 schematically shows the support structure of a housing of the invention, with covering segments partly disposed thereon;

FIG. 5 schematically shows the mode of securing a covering segment to the support structure of a housing of the invention;

FIG. 6 shows the overlap between covering segments abutting one another in the vertical direction;

FIG. 7 is an enlarged detail of a housing of the invention in the region of the rotor shaft leadthrough, to illustrate the overlap in this region;

FIG. 8, in an enlarged view, shows a detail to illustrate the overlap at the intersection of two roof segments of the housing of the invention;

FIG. 9, in an enlarged view, shows a region of the overlap between two covering segments in the vicinity of the roof of the housing; and

FIG. 10 in a schematic illustration shows the disposition of a sealing lip to seal off the transition region between two covering segments.

WAY(S) FOR EMBODYING THE INVENTION

In the drawings, in schematic basic views that are not to scale, individual views are shown of one exemplary embodiment for a housing of the invention for the nacelle of a wind turbine. These drawings and the description below of the exemplary embodiment shown in them are intended to promote understanding of the invention in its general scope and are not limiting.

In FIGS. 1 and 2, the basic construction of a housing 1, constructed according to the invention from one support structure and a plurality of covering segments, for the nacelle of a wind turbine is shown in two different views. The housing 1 may also be called a machine house covering. On a support frame, not shown in detail in these drawings, a total of twelve covering segments are disposed and affixed; they are embodied as self-supporting and together form the housing 1. These segments are divided up into two ceiling segments 2, which as elongated components extend over the entire length of the housing 1 and abut one another approximately in the middle of the housing cap or ceiling with a seam extending longitudinally; a total of four side wall segments 3, of which two segments each, disposed vertically one above the other and extending over the entire length of the housing 1, cover the right and left side wall of the housing 1, respectively; two front segments 4; two rear segments 5; and two bottom segments 6, again extending over the entire length of the housing 1, which likewise abut one another at a seam point extending longitudinally approximately in the center of the bottom.

A through opening 7, through which the rotor shaft is passed and on which the hub with the rotor blades is seated outside the housing 1, is left by the front segments 4.

A further opening 8 is left by the bottom segments 6. Via this opening 8, the housing 1 is connected to the tower of a wind turbine, via a rotatable connection for adjusting the azimuth angle.

As already noted, an essential aspect of the invention is that the housing 1 is constructed of a support structure and self-supporting elements, secured to it, that have a plastic surface, which in this exemplary embodiment is preferably a FRP surface. One example for how a covering element can be constructed to be self-supporting is shown in a schematic view in FIG. 3. In FIG. 3, as an example, one side wall segment 3 is shown; the mode of construction shown here and the construction shown can fundamentally be adopted for the other covering segments, the ceiling segment 2, front segment 4, rear segment 5, and bottom segment 6.

The side wall segment 3 in FIG. 3 first includes a frame 9, which can also be called a scaffolding construction. This frame 9 is formed here of metal braces or tubes; besides encompassing longitudinal braces 10 and transverse braces 11, stave-like connecting braces 12 are provided for stabilizing and making the attachment possible. On this frame 9, or this trellis formed of the frame 9 together with the connecting braces, a shell element 13 of FRP is placed and solidly connected to the frame 9. The shell element 13 comprises a single FRP plate, kept thin overall with a thickness of approximately 4 mm, which by itself is not yet inherently stable or self-supporting. Only upon being connected to the frame 9 does the thus-formed covering segment, in the form of the side wall segment 3, gain its self-supporting property.

For connecting the shell element 13 to the frame 9, overlaps 14 that in tunnel-like fashion cover the longitudinal braces 10 of the frame 9 are formed from FRP. Between the longitudinal braces 10 of the frame 9 and the overlaps 14, there is a play such that a relative motion between the frame 9 and the shell element 13 is possible, in particular a relative motion caused by different thermal expansions. Since such thermal expansions are the most relevant in the direction of the longest extent, the tunnel-like overlaps 14 extend in precisely this longitudinal direction, and the transverse braces 11 and the connecting braces 12 are not held by overlaps or firmly connected to the shell element 13. The overlaps 14 are expediently also made from FRP and in particular can already have been formed in the production process of the shell element 13. In this phase, the frame 9 can be laminated into the shell element 13, to form the self-supporting covering segment (side wall element 3).

In the exemplary embodiment shown here, the frame 9 is formed of square metal tubes, but instead of such metal tubes, equivalent structures of plastic, in particular FRP, can also be used. The use of FRP of the same kind as that of the shell element 13 has the advantage that because the material properties are the same, differences in thermal expansion need not be expected, and thus that no compensatory provisions have to be made here. On the other hand, a basic metal frame can be made dimensionally more stable by simple means than can comparable FRP elements.

In FIG. 4, it is shown in a detail how in a housing of the invention, the individual covering segments, in this case side wall segments 3, roof segments 2 and a front segment 4, are affixed to a support structure 15, of which three vertically oriented curves are shown. The affixation is done in a detachable way, with conventional fastening mechanisms, in particular screw means. The connecting braces 12 of the frames 9 that form the intrinsic stability of the covering segments are disposed in such a way that when the particular covering segment is correctly attached to the curves of the support structure 15 they come to rest adjoining the latter and can be affixed there with suitable means, in particular screwed there.

In this example, the support structure is formed of metal braces or tubes, but it can also comprise plastic, in particular FRP.

In FIG. 5, this attachment of the frame 9 of a covering segment to the support structure 15 is shown once again in a schematic illustration. On the left in the drawing is the front end of the housing, in which the through opening 7 is located once the housing is complete. A bracket 16 is formed on this front end, on the next closest curve of the support structure 15, and on this bracket the frame rests with a connecting brace 12 and is screwed there. This screwing provides an exact fixation in the z direction as shown in the drawing. The frame is screwed to the next curves, in the direction of the rear end of the housing, of the support structure 15 via respective oblong slots 17, which on the one hand make it possible to compensate for production tolerances and on the other can absorb dimensional differences that can arise between the support structure 15 and the frame 9 because of different thermal expansions.

In FIG. 6, in a view from the front and without the front segment, a detail is schematically shown of the housing 1 of the exemplary embodiment. In this embodiment, it is important to recognize the regions of the overlaps 18, in each of which the upper end of a lower-located covering segment is guided underneath the abutting end of a higher covering segment, in order to form an air gap and at the same time to provide a secure seal from rainwater or splashing water that may be running off.

One such overlap between the ceiling segment 2 and a side wall segment 3 is shown again in FIG. 8, further enlarged and more clearly apparent. It can be seen especially well here that in the vicinity of the overlap 18, the upper end 19 of the lower covering segment, in this case the side wall segment 3, springs back and is guided beneath the end 20 of the upper covering segment, in this case the ceiling segment 2. The end 20 of the ceiling segment 2 is aligned with the remainder of the course of the shell element 13 of the side wall segment 3. In the vicinity of the overlap, an air gap 21 between the ceiling segment 2 and the side wall segment 3 remains, which can amount to from 5 to 20 mm, in particular 10 to 15 mm, and serves in particular to reduce the tolerance requirements for dimensional stability of the shell elements 13 of the ceiling segment 2 and of the side wall segment 3. In a housing of the invention, such air gaps are left at all the abutting points between two covering segments.

In FIG. 7, a further abutting point is shown, in this case the substantially vertical abutting point between one side wall segment 3 and one front segment 4. In the vicinity of this abutting point, the shell element 13 of the side wall segment 3 is bent three times overall to form an approximately U-shaped channel 22, which in turn is covered by a bent-over edge 23 of the front segment 4. Once again, an air gap can be seen between the bent-over edge 23 of the front segment 4 and the part, extending in alignment with this edge, of the shell element 13 of the side wall segment 3. The U-shaped channel 22 not only provides watertightness but also forms a kind of labyrinth seal, which at least to a certain extent helps to prevent dirt particles, sand, and the like from penetrating into the interior of the housing.

In a similar way, a kind of labyrinth seal is embodied at the seam point between two ceiling elements 3, as is shown in FIG. 9. Here, the shell element 13 of the ceiling segment 2, shown on the left in the drawing, protrudes with an end region 24 bent three times to form a kind of tunnel, past the upward-placed end 25 of the shell element 13 of the ceiling element 2 shown at the right in the drawing.

To achieve even better sealing off of the air gaps, particularly in regions with a greater input of harmful particles, such as abrasive sand particles, corrosive salt particles, or the like, sealing lips, for instance of rubber, can be inserted into these air gaps; the sealing lips do continue to leave the air gap, visible in FIG. 9, and thus allow the possibility of specifying more generous tolerances for the fabrication of the shell elements 13, but they still ensure a safe amount of sealing. This is shown in a highly schematic view in FIG. 10, taking as the example the abutting point between two side wall segments 3. Here, a sealing lip 25 is placed in the vicinity of the overlap 18 onto the lower one of the side wall segments 3; the sealing lip rests on the upper side wall segment 3 and thus ensures sealing off of this gap space.

LIST OF REFERENCE NUMERALS

-   -   1 Housing     -   2 Ceiling segment     -   3 Side wall segments     -   4 Front segment     -   5 Rear segment     -   6 Bottom segment     -   7 Through opening     -   8 Opening     -   9 Frame     -   10 Longitudinal brace     -   11 Transverse brace     -   12 Connecting brace     -   13 Shell element     -   14 Overlap     -   15 Support structure     -   16 Bracket     -   17 Oblong hole     -   18 Overlap     -   19 End     -   20 End     -   21 Air gap     -   22 Channel     -   23 Bent-over edge     -   24 End region     -   25 End     -   26 Sealing lip 

1. A housing for the nacelle of a wind turbine, having a support structure comprising tubes or braces, and covering segments affixed to the support structure wherein the covering segments have a covering surface of plastic, and are embodied as self-supporting.
 2. The housing as defined by claim 1, wherein the covering segments each have a reinforcing frame and each have a shell element of plastic, placed two-dimensionally on this frame on a frame face, and the frames are connected to the support structure.
 3. The housing as defined by claim 2, wherein the covering segments are composite components, with frames of metal tubes or braces, onto which the shell elements are placed, and the frames are connected to the support structure via construction means.
 4. The housing as defined by claim 2, wherein the frames of the covering segments comprise plastic.
 5. The housing as defined by claim 2 wherein the shell element and the respective associated frame are connected by overlaps of the plastic material via parts of the frame, and at least in regions serving to connect the frame to the support structure, parts of the frame are exposed.
 6. The housing as defined by claim 5, wherein the overlaps are embodied such that relative motions between the frame and the shell element are enabled.
 7. The housing as defined by claim 2 wherein the frames of the covering segments are fabricated as closed, encompassing frames.
 8. The housing as defined by claim 1 wherein between the covering segments, at abutting points, air gaps with a gap width of 5 mm to 20 mm are left.
 9. The housing as defined by claim 8, wherein a first covering segment, disposed in a fall line of the housing above a second covering segment, protrudes with an edge past an edge of the second covering segment.
 10. The housing as defined by claim 8 wherein sealing lips for closing the air gaps are disposed with at least at some of the abutting points.
 11. The housing as defined by claim 1 wherein covering surface portions of the covering segments have smooth surfaces, with a thickness of less than 10 mm.
 12. The housing as defined by claim 1 wherein the support structure has a lower carrier portion, extending in the longitudinal direction of the housing, and at least two portals, spanning this support structure in the transverse direction of the housing, to which portals at least lateral and upper covering segments are affixed.
 13. The housing as defined by claim 1 wherein at least one of the covering segments is affixed detachably to the support structure.
 14. The housing as defined by claim 1 wherein a total of twelve covering segments are disposed, two each for covering a top side, an underside, and rear, left, front and back face ends of the nacelle.
 15. The housing as defined by claim 1 wherein the size of the covering segments is dimensioned such that, they can be received in a 40-foot container.
 16. The housing as defined by claim 3 wherein the shell element and the respective associated frame are connected by overlaps of the plastic material via parts of the frame, and at least in regions serving to connect the frame to the support structure, parts of the frame are exposed.
 17. The housing as defined by claim 4 wherein the shell element and the respective associated frame are connected by overlaps of the plastic material via parts of the frame, and at least in regions serving to connect the frame to the support structure, parts of the frame are exposed.
 18. The housing as defined by claim 16, wherein the overlaps are embodied such that relative motions between the frame and the shell element are enabled.
 19. The housing as defined by claim 17, wherein the overlaps are embodied such that relative motions between the frame and the shell element are enabled.
 20. The housing as defined by claim 3 wherein the frames of the covering segments are fabricated as closed, encompassing frames. 